Currently used arrangements for producing circularly polarized light are described, for example, at page 42 of a textbook entitled "Introduction to Modern Optics" by Grant R. Fowles, published by Holt, Rhinehart & Winston in 1968. As shown therein the arrangement comprises a linear polarizer having its transmission axis positioned at a 45.degree. angle to the orthogonal fast and slow axes of a quarter waveplate through which the light passes after passing through the linear polarizing crystal.
Such a circular polarizer has two major drawbacks. First, the extinction ratio for such devices can have an acceptably high value only for a narrow bandwidth of light wavelengths. By the "extinction ratio" is here meant the ratio of the light intensity transmitted by two sequentially positioned polarizers that each pass polarization of the same circular polarization handedness sense to the light intensity transmitted by two such polarizers which respectively pass circular polarization handedness senses which are opposite. Outside of the narrow bandwidth for which known circular polarizers have a high extinction ratio, the ratio drops fast. This characteristic is intrinsic to the presently available devices because, as noted, they are constructed from a linear polarizer and a quarter waveplate. Naturally, the waveplate can be a quarter waveplate only for one wavelength. For other wavelengths the plate retardation in terms of the wavelength changes proportionally to the reciprocal of the wavelength. Thus, the light instead of being circularly polarized is elliptically polarized. Circular polarizers for the visible wavelength region have extinction ratios at the two extremes of the visible region of 5 or 6 at 400 nm and of 6 or 7 at 700 nm compared to 1000 at 550 nm.
The second major drawback of known absorption type circular polarizers is that they absorb between 60% and 80% of the incident unpolarized light. This is a problem not only because the efficiency of the resulting light source is thereby low, but also because the dissipated power in the polarizer, in the case of a high intensity beam, damages the polarizer.
In view of these problems, systems such as that disclosed, for example, in U.S. Pat. No. 2,958,258 to D. H. Kelly or U.S. Pat. No. 3,893,758 to Hunzinger et al, have used linearly polarized light rather than circularly polarized light in high intensity projection systems requiring polarized light sources. Such systems may, for example, use a wideband linear polarizer of the type shown in U.S. Pat. No. 3,403,731, issued to S. M. MacNeille, in order to obtain the wideband or white polarized light source needed for the system. Although the MacNeille device solves the problem of providing a wideband rather than a narrow band polarizer for linearly polarized light, it does not purport to be useful for providing circularly polarized light which may often be more desirable than linearly polarized light for various system requirements.
There have been proposals in the literature to use alternately left-handed and right-handed cascaded cholesteric liquid crystal cells to form narrow band or notch color filters. Such a proposal was made, for example, by James Adams et al in an article entitled "Cholesteric Films as Optical Filters" which appeared in Vol. 42, No. 10, of the September 1971 issue of the Journal of Applied Physics beginning at page 4096. See also an article by Sato et al entitled "Liquid Crystal Color Light Valve" appearing in the February 1974 issue of the IEEE Transactions on Electron Devices at page 171. These devices utilize the polarizing action of liquid crystal materials. However, they do not form wideband circular polarizers which will take a source of unpolarized white light and transform it into a beam of circularly polarized white light. Furthermore, even at narrow bandwidths these devices inherently use only 50% of the incident unpolarized light since their function by definition is to transmit 50% of the incident light as circularly polarized light in a first handedness sense (left-handed or right-handed as the case may be) and to reflect the other 50% of the incident unpolarized light as circularly polarized light of the opposite handedness sense.
It is an object of the present invention to overcome the foregoing shortcomings of the prior art and to provide a circularly polarized light source using one or more cholesteric liquid crystal cell means to achieve 100% nominal efficiency in the use of the incident light at any bandwidth rather the 50% shown in the prior art and/or to achieve a wideband circular polarizer capable of producing an output beam of white circularly polarized light.